1. Field of the Invention
This invention relates to a gray conversion method and a gray conversion apparatus for carrying out a gray conversion on image data representing a color image in order to output a color image having a halftone to a monochromatic printer.
2. Description of the Related Art
Recently almost all the digital images are color images having a halftone part owing to advance in image processing, reduction in price of data storage devices such as memories and hard discs, and the like. Though color printers are being popularized, color images having a halftone are often output through a monochromatic printer. Further color images are sometimes output through a monochromatic printer for a special effect. When image data representing a color image having a halftone is input into the monochromatic printer as it is, the print output through the monochromatic printer generally becomes too strong in contrast and becomes blackish or whitish. Accordingly, in order to improve finish of the print, it is necessary to convert the color image data to halftone image data having information on density of pixels and a proper xcex3-characteristic.
In the conversion process for converting color image data to density information data, gray conversion is a very important process which governs finish of the print.
The gray conversion is generally effected by xcex3-correction or color adjustment (e.g., RGB ratio adjustment). It is preferred that both the xcex3-correction and the color adjustment be carried out as the gray conversion. However, there has been developed no image application software which carries out both the xcex3-correction and the color adjustment though there has been developed image application software which carries out only the xcex3-correction or the color adjustment.
When an image is reproduced by a monochromatic printer on the basis of color image data, finish of the prints cannot be controlled by the printer. Accordingly, the color image data should be input into the printer after subjected to the gray conversion. Though the gray conversion may be carried out by a printer driver, there has not been developed a printer driver which carries out the RGB ratio adjustment which greatly affects the result of the gray conversion. Though there has been known a printer driver which carries out the xcex3-correction, the result of the xcex3-correction cannot be known unless prints are made on the basis of the corrected image data.
In view of the foregoing observations and description, the primary object of the present invention is to provide a gray conversion method and a gray conversion apparatus which permits the operator to carry out RGB ratio adjustment on color image data control as the gray conversion while visually recognizing the result of the RGB ratio adjustment so that the color image data can be effectively and properly converted to halftone image data having information on density of pixels.
In accordance with a first aspect of the present invention, there is provided a gray conversion method for converting original color image data to halftone image data having information on density of pixels by a predetermined gray conversion, the halftone image data being to be input into a monochromatic printer when a color image represented by the original color image data is to be printed by the monochromatic printer, the method characterized in that
said predetermined gray conversion includes an RGB ratio adjustment for setting the ratio of R, G and B in the original color image data to an optimal value,
an RGB ratio adjustment is carried out on reference image data (e.g., image data derived from the original color image data or image data representing a color map) employing different ratios of R, G and B,
the results of the predetermined gray conversions including the RGB ratio adjustments employing the different ratios of R, G and B carried out on the reference image data are graphically shown on a screen,
the optimal value of the ratio of R, G and B is determined on the basis of comparison of the results of the predetermined gray conversions graphically shown on the screen, and
the predetermined gray conversion including an RGB ratio adjustment employing the optimal value of the ratio of R, G and B is carried out on the original color image data.
The reference image data may be, for instance, image data derived from the original color image data or image data representing a color map.
The xe2x80x9cimage data derived from the original color image dataxe2x80x9d may be any image data derived from the original color image data and may be the original color image data as it is. However it is preferred in view of the facility of gray conversion that the image data derived from the original color image data be image data representing a reduced original image.
The xe2x80x9ccolor mapxe2x80x9d is obtained by arranging, in a pattern of a pallet, colors obtained by thinning, at a predetermined ratio, colors generated by all the combinations of densities of R, G and B, and does not depend upon the original color image data to be processed.
The RGB ratio adjustment is to adjust the ratio of coefficients xcex1, xcex2 and "khgr" in the following formula.
Gray=xcex1Red+xcex2Green+"khgr"Blue 
It is preferred that the gray conversion method in accordance with the first aspect of the present invention be carried out by a utility which is positioned between image application software and a printer driver and operates in combination with the printer driver, a printer driver, or a print control system of an OS (operating system).
It is further preferred that said predetermined gray conversion includes, in addition to the RGB ratio adjustment, a xcex3-correction in which the xcex3-curve and/or the xcex3-value is adjusted, a xcex3-correction is carried out on the reference image data employing different xcex3-curves and/or xcex3-values, the results of the xcex3-correction carried out on the reference image data employing the different xcex3-curves and/or xcex3-values are graphically shown on a screen, the optimal xcex3-curve and/or the xcex3-value is determined on the basis of comparison of the results of the xcex3-correction graphically shown on the screen, and the predetermined gray conversion including an RGB ratio adjustment employing the optimal value of the ratio of R, G and B and a xcex3-correction employing the optimal xcex3-curve and/or the xcex3-value is carried out on the original color image data.
In accordance with a second aspect of the invention, there is provided a gray conversion apparatus comprising a gray conversion means which carries out a predetermined gray conversion on original color image data to convert the original color image data into halftone image data having information on density of pixels, the halftone image data being to be input into a monochromatic printer when a color image represented by the original color image data is to be printed by the monochromatic printer, the apparatus characterized in that
the gray conversion means includes
an RGB ratio adjustment means which carries out an RGB ratio adjustment for setting the ratio of R, G and B in the original color image data to an optimal value and carries out an RGB ratio adjustment on reference image data (e.g., image data derived from the original color image data or image data representing a color map) employing different ratios of R, G and B,
a graphic display means which graphically shows the results of the predetermined gray conversions including the RGB ratio adjustments employing the different ratios of R, G and B carried out on the reference image data on a screen, and
an RGB ratio determining means which determines the optimal value of the ratio of R, G and B on the basis of comparison of the results of the predetermined gray conversions graphically shown on the screen, and
the gray conversion means carries out the predetermined gray conversion including an RGB ratio adjustment employing the optimal value of the ratio of R, G and B determined by the RGB ratio determining means on the original color image data.
It is preferred that the gray conversion means be in the form of a utility which is positioned between image application software and a printer driver and operates in combination with the printer driver, in the form of a printer driver, or in the form of a print control system of an OS.
It is further preferred that said gray conversion means further comprises
a xcex3-correction means which carries out a xcex3-correction on the reference image data employing different xcex3-curves and/or xcex3-values,
a graphic display means which graphically shows the results of the xcex3-corrections carried out on the reference image data employing the different xcex3-curves and/or xcex3-values on a screen, and
a xcex3-correction condition determining means which determines the optimal xcex3-curve and/or the xcex3-value on the basis of comparison of the results of the xcex3-correction graphically shown on the screen, and
the gray conversion means carries out the predetermined gray conversion including an RGB ratio adjustment employing the optimal value of the ratio of R, G and B and a xcex3-correction employing the optimal xcex3-curve and/or the xcex3-value on the original color image data.
In accordance with the present invention, since the influence of the RGB ratio adjustment on the gray conversion is graphically shown on a screen, the result of the gray conversion can be known without making prints on the basis of a selected ratio of R, G and B, whereby a gray conversion including an RGB ratio adjustment can be efficiently carried out on color image data.
Further when the gray conversion is carried out by a utility which is positioned between image application software and a printer driver and operates in combination with the printer driver, a printer driver, or a print control system of an OS, the gray conversion can be carried out independently of the image application software.
Further when the predetermined gray conversion includes a xcex3-correction in addition to the RGB ratio adjustment, the results of the xcex3-correction carried out on the reference image data employing the different xcex3-curves and/or xcex3-values are graphically shown on a screen, the optimal xcex3-curve and/or the xcex3-value is determined on the basis of comparison of the results of the xcex3-correction graphically shown on the screen, and the predetermined gray conversion including an RGB ratio adjustment employing the optimal value of the ratio of R, G and B and a xcex3-correction employing the optimal xcex3-curve and/or the xcex3-value is carried out on the original color image data, the result of the gray conversion can be known without making prints on the basis of a selected ratio of R, G and B and a selected xcex3-curve and/or xcex3-value, whereby a gray conversion including an RGB ratio adjustment and a xcex3-correction can be efficiently carried out on the color image data.